Techno-Economic Analysis of a Novel Biofilm-Based Algae Cultivation System for Integrated CO2 Capture and Aquaculture Wastewater Treatment

Abstract

The rapid growth of industrial activity and global energy demand has intensified greenhouse gas emissions, making carbon capture and sustainable waste management two of the most pressing priorities of our time. The industrial sector, which accounts for a large share of total U.S. greenhouse gas emissions, generates CO₂-rich flue gas streams from energy-intensive operations such as kraft pulp mills, where lime kilns alone produce flue gas with high CO₂ concentrations. Simultaneously, the southeastern United States hosts a thriving aquaculture sector that generates large volumes of nutrient-laden wastewater, creating both an environmental burden and an underutilized resource. Microalgae offer a biological platform to address both challenges simultaneously, fixing industrial CO₂ through photosynthesis while assimilating nitrogen and phosphorus from wastewater to produce biomass with commercial value as aquafeed. However, the high cost of conventional suspended cultivation, driven by energy-intensive harvesting and poor CO₂ utilization, has long hindered commercially viable large-scale deployment. This thesis presents the dry biofilm photobioreactor (DBP) as an emerging attached-growth technology that addresses many of these limitations by growing microalgae as a semi-dry biofilm on belt substrates within an enclosed greenhouse and a comprehensive techno-economic analysis (TEA) for this system. In Chapter 1, the need for this research and the rationale for the proposed system are established. The chapter examined the limitations of conventional microalgae cultivation technologies, open raceway ponds, and enclosed photobioreactors. A novel dry biofilm photobioreactor (DBP) is introduced as the proposed solution. In the DBP, microalgae grow as a semi-dry biofilm on a moving polymer belt within a greenhouse enclosure. The biofilm surface is directly exposed to the CO₂-rich flue gas, eliminating the liquid-film resistance that limits carbon delivery in conventional systems. Nutrients are supplied by intermittently misting aquaculture wastewater onto the biofilm, while harvesting is achieved by mechanically scraping the biomass, removing the need for centrifugation or flocculation. The chapter concludes by identifying the research gap: despite the promising design concept, no rigorous techno-economic analysis existed to evaluate whether the DBP system can compete economically at a commercial scale. Chapter 2 describes the methodology used to evaluate the DBP system. The analysis was structured around a 1,000-module (10 greenhouses) commercial-scale facility co-located with a kraft pulp mill, using lime kiln flue gas (20 vol% CO₂) and aquaculture wastewater as zero-cost process inputs. Two operating scenarios were evaluated: a 24-hour LED-supplemented scenario and a 12-hour natural sunlight-only scenario. The economic model used the discounted cash flow rate of return (DCFROR) method under nth-plant assumptions, consistent with published algal techno-economic studies, to calculate the minimum biomass selling price (MBSP) as the primary economic metric. Chapter 3 presents and discusses the results of techno-economic analysis. Mass balance results showed that the 24-hour scenario produced more than double the throughput of the 12-hour scenario. Both scenarios achieved approximately 99% nitrate removal and complete phosphate removal from the aquaculture wastewater, meeting typical discharge standards. Total capital investment and annual operating expenditure were calculated for both scenarios. Sensitivity analysis confirmed that biomass productivity was the dominant cost driver in both cases. Seasonal analysis of the 12-hour scenario revealed a 30–40% MBSP swing between summer and winter conditions driven by daylength variation. Scenario analyses further quantified the economic value of natural sunlight supplementation over fully artificial LED illumination, and the thermal co-location benefit of using waste steam from the kraft mill in place of natural gas for drying. In Chapter 4, the findings are gathered, and their broader significance is discussed. The DBP system demonstrated technically promising and economically competitive performance for the simultaneous capture of industrial CO₂, recovery of nutrients from aquaculture wastewater, and production of microalgal aquafeed. The 24-hour scenario is suited to large-scale commercial operations where maximum biomass throughput and low-cost electricity are available, while the 12-hour scenario offers a lower capital- and energy-intensity pathway for early-stage deployment or cost-sensitive markets. The chapter also identified the key limitations of this work, primarily that productivity values were adapted from conventional biofilm literature rather than measured directly on the DBP system. Since these results come from a modeling study, real-world applications will require pilot-scale validation under actual flue-gas conditions before commercial deployment. Future research priorities include pilot-scale experimental validation, LED photoperiod optimization, and an integrated life cycle assessment to fully characterize the environmental footprint alongside the economic case.